1. Field of the Invention
This invention relates generally to a method for the catalytic purification of crude isophthalic acid and to the catalyst system employed therein, and more particularly concerns the use in such purification of a catalyst bed comprising Group VIII noble metal components comprising at least two of palladium-, platininum-, rhodium-, ruthenium-, osmium-, and iridium-containing components.
2. Discussion of the Prior Art
Polymer grade or "purified" isophthalic acid is one of the starting materials which are employed in the manufacture of unsaturated polyesters. Purified isophthalic acid is derived from relatively less pure, technical grade or "crude" isophthalic acid by purification of the latter utilizing hydrogen and a noble metal catalyst, of the type described in Meyer, U.S. Pat. No. 3,584,039 or Stech et al., U.S. Pat. No. 4,405,809 for the purification of crude terephthalic acid. In the purification process, the crude isophthalic acid is dissolved in water at an elevated temperature, and the resulting solution is hydrogenated, preferably in the presence of a hydrogenation catalyst containing a noble metal, typically palladium, on a carbon support, as described in Pohlmann, U.S. Pat. No. 3,726,915 for the purification of crude terephthalic acid. This hydrogenation step converts the various color bodies present in the crude isophthalic acid to colorless products.
However, even after the aforesaid purification, the purified isophthalic acid product contains color bodies. Therefore, it is highly desirable to reduce the concentration of such color bodies that remain in purified isophthalic acid. The color level of purified isophthalic acid product is generally measured directly either by measuring the optical density of solutions of purified isophthalic acid or the b*-value of the solid purified isophthalic acid itself. The optical density of purified isophthalic acid is measured as the absorbance of light at 340 and 400 nanometers (nm) in its basic solution in a solvent such as sodium hydroxide or ammonium hydroxide.
The measurement of the b*-value of a solid on the Hunter Color Scale is described in Hunter, The Measurement of Appearance, Chapter 8, pp. 102-132, John Wiley & Sons, N.Y., N.Y. (1975), and in Wyszecki et al., Color Science, Concepts and Methods, Quantitative Data and Formulae, 2d Ed., pp. 166-168, John Wiley & Sons, N.Y., N.Y. (1982).
More specifically, the b*-value of purified isophthalic acid can be determined using, for example, a Diano Match Scan Spectrophotometer as follows. Purified isophthalic acid is pressed into a pellet having a thickness of about 0.25 inch and a diameter of about 1 inch. The pellet is then irradiated with white light that has been UV-filtered. The spectrum of the visible light reflected from the sample is determined and tristimulus values (X, Y, and Z) are computed using the CIE Standard Observer functions. Using the weighted-ordinate method, tristimulus values are obtained from the following equations: ##EQU1## where R.sub..lambda. is the percent reflectance of the object at wavelength .lambda. and x.sub..lambda., y.sub..lambda., and z.sub..lambda. are the Standard Observer functions at wavelength .lambda. for CIE Illuminant D65. The tristimulus values, X, Y and Z, identify the color of the object in terms of the mixture of the primary lights that match it visually. Tristimulus values, however, are of limited use as color specifications, because they do not correlate with visually meaningful attributes of color appearance and are not uniform in the spacing of colors as related to visual differences. As a result, "Uniform Color Scales" (UCS) have been adopted which use simple equations to approximate visual response. The UCS scale used by the Diano instrument is the CIE 1976 L*a*b* formula which converts tristimulus values to L*, a*, and b* values as shown below: EQU L*=25(100Y/Y.sub.o).sup.1/3 -16 EQU a*=500[(X/X.sub.o).sup.1/3 -(Y/Y.sub.o).sup.1/3 ] EQU b*=200[(Y/Y.sub.o).sup.1/3 -(Z/Z.sub.o).sup.1/3 ]
The L*-value is a measure of the luminosity or whiteness of an object where L*=100 is pure white, L*=0 is black, and in between is gray. The L*-value is strictly a function of the tristimulus Y-value. The b*-value is a measure of the yellowness-blueness attribute where positive b*-values represent yellow appearance and negative b*-values represent blue appearance. The b*-value is a function of both tristimulus values Y and Z.
Furthermore, even after purification, the purified isophthalic acid product often contains impurities which fluoresce at wavelengths of about 390 and 400 nanometers (nm) upon excitation at wavelengths of 260-320 nanometers. Further reduction of such fluorescence of the purified isophthalic acid product is highly desirable. Since the concentration of such impurities in purified isophthalic acid can vary significantly, specifications are often established for the amount of such fluorescence which can be permitted for the purified isophthalic acid product. The problem of the control of such fluorescence by purified isophthalic acid is complicated because some of the fluorescent impurities are soluble and can be removed by conventional procedures for purifying isophthalic acid while other such fluorescent impurities are insoluble and cannot be removed by such conventional procedures. Furthermore, upon chemical reduction during purification of crude isophthalic acid, some impurities which do not themselves fluoresce at wavelengths of 390 and 400 nanometers upon excitation at wavelengths of 260-320 nanometers are converted to their reduced forms which fluoresce at 390 and 400 nanometers upon excitation by wavelengths of 260-320 nanometers.
Puskas et al., U.S. Pat. Nos. 4,394,299 and 4,467,110 disclose the use of a combination noble metal catalyst, for example, a palladium/rhodium catalyst on a porous carbonaceous surface, for purification of aqueous terephthalic acid solutions. These two patents also show the use of a rhodium-on-carbon catalyst under reducing conditions and review various heretofore known methods of preparing a Group VIII metal catalyst having activity and selectivity suitable for the purification of terephthalic acid by hydrogenating its principal impurity, 4-carboxybenzaldehyde, to p-toluic acid.
We have now discovered that the use in the aforesaid purification of crude isophthalic acid of a catalyst system comprising metal components comprising at least two of palladium-, platinum-, rhodium-, ruthenium-, osmium- and iridium-containing components supported on active carbon carrier particles, and the passage of the aqueous solution of crude isophthalic acid through a bed of the aforesaid catalyst particles effects a further decrease in the concentration of color bodies and of fluorescent impurities in the resulting purified isophthalic acid, relative to the use of a conventional palladium-on-carbon catalyst alone.